This invention relates to systems that ascertain what is occupying a vehicle seat for the purpose of deciding how to best protect an occupant.
Air bags of occupant protection systems are expensive and in certain circumstances are dangerous. It is therefore important to avoid deployment when the seat is empty to save the cost of replacement. It is important to avoid deployment when circumstances do not warrant deployment or when deployment might do more harm than good. It is particularly important to deploy the airbag judiciously when the seat is occupied by a child or by a very small adult. A system is desired to reliably distinguish a 105 pound adult from a child even when the child is in a child seat and belts retaining the child seat are under substantial tension.
Occupant protection systems typically include a xe2x80x9csensor and diagnostic modulexe2x80x9d or xe2x80x9cSDMxe2x80x9d which performs various functions related to sensing the severity of a vehicle crash, monitoring various elements of the occupant protection system for proper operation, and initiating deployment of occupant protection means. SDMs typically include a microprocessor, an accelerometer, an arming sensor, circuitry interconnecting the aforementioned components and switches for initiating deployment of the occupant protection devices. SDMs may be connected to receive input from such as side mounted and forward mounted crash sensors.
Knowledge of the weight of a seat occupant is useful. If the weight is very small it may be assumed that the seat is unoccupied or occupied by a small child; in either case airbag deployment would not be desired. If the weight is intermediate, say between 30 and 45 kilograms, then the occupant is likely to be a child and whether or not an airbag should be deployed depends on factors such as how energetically the airbag deploys. If the weight is greater than 45 kilograms the seat occupant is likely to be an adult who would be protected by an airbag.
Three types of weight sensing systems for installation in vehicle seats are known: A first type of weight sensing system comprises an array of force sensors located immediately beneath the upholstery material of the seat cushion which operates to measure the pressure of the occupant against the seat at the points where sensors are located. These sensors are typically responsive to small forces applied over a small area and an array of force sensors tells a microprocessor the magnitude and distribution of the force the occupant applies to the cushion. The microprocessor ascertains the weight and other attributes of the seat occupant from the information provided by the array of force sensors.
The second type of weight sensing system is useful in the type of seat having a seat cushion supported by a platform. The second type of weight sensing system includes, typically, four force sensors located at the four corners of the platform where they can transfer force to the frame of the seat. The outputs of the four sensors are added to ascertain the total weight being supported by the platform and, therefore, by the seat cushion.
The third type of weight sensing system comprises sensors for sensing stress in structural members of the seat. A chair having a load cell at each leg for sensing the weight carried by the leg with an adder for adding the inputs from each load cell would be an example of the third type of weight sensing system. The outputs of the sensors (four load cells in the case of the aforementioned chair) are added and the weight of the empty seat is subtracted to obtain the weight of the occupant.
The known embodiments of the aforementioned weight sensing systems do not always measure the occupant""s weight accurately and no design is widely accepted. Certain of the aforementioned weight sensing systems may provide weight readings for a tightly belted child seat that resemble weight readings for an adult.
Load cells comprising a piston sealingly movable in a cylinder to generate hydraulic pressure are well known. At the front of a reclined seat the seat may apply upward force to a load cell which requires a load cell that responds to both tension and compression. To measure tension, a sensor based on a piston sealingly movable in a tube must be preloaded by such as a spring to maintain a pressure in the liquid that diminishes when tension is applied. The output of load cells preloaded by springs may vary with temperature because liquids typically have larger thermal expansion coefficients than metals, which leads to varying spring deflection with temperature and, therefore, varying preload with temperature. A gel is often used as the liquid because it is easier to seal against leakage.
Load cells comprising a piston sealingly movable in a cylinder have friction between the piston and the cylinder when there are side forces. There are many causes for side forces. In a vehicle side forces can be caused by differential thermal expansion between the car floor and the seat, forces caused by attaching the seat to the vehicle, damage to the seat or the car floor and forces resulting from acceleration of the vehicle or actions of the seat occupant. It is important to isolate the piston from angular misalignment between seat parts and car floor parts that occur because of production variations in the parts. A load cell is needed that is inherently insensitive to side forces and angular misalignments.
Seat occupant weight sensing systems responsive to stress in the seat structure must respond only to forces resulting from the weight of the seat occupant and not to stresses resulting from thermal expansion or attachment to the vehicle. An advantage of seat occupant weight sensing systems responsive to stress in the seat structure is that they present a solution to the aforementioned problem of belt forces causing a child to appear to be an adult. Anchoring the seat belts to the seat frame and placing the force sensors between the belt anchors and the vehicle attachment points makes the measured weights independent of belt forces.
It is often desired to place four load cells between the vehicle floor and the seat. There are times when substantial forces can occur between a seat and the vehicle floor. For example, if a structural member of a seat is attached to the floor of a vehicle it can happen that the structural member remains at a temperature comfortable to the vehicle occupants while the vehicle floor goes from a very cold temperature caused by winter conditions to a very high temperature caused by heat rising from a catalytic converter. The result is relative thermal expansion of the floor that can cause substantial horizontal stresses that will be experienced by load cells placed between the floor of a vehicle and the seat.
Semiconductor pressure sensors are manufactured in large quantities by micromachining silicon wafers. Designs are based on various technologies and physical principles. These sensors may require additional components to meet needs for such as temperature compensation. Typically, but not necessarily, a second circuit assists the micromachined pressure sensing element. Certain micromachined sensors operate immersed in the liquid as they sense the pressure of the liquid.
The aforementioned need for temperature compensation and other needs such as compensation for nonlinear pressure response and variable overall span are typically met by including an inexpensive microprocessor or an xe2x80x9capplication specific integrated circuitxe2x80x9d (ASIC), which is a purpose built microprocessor, in close proximity to the pressure sensor.
Load cells of the type that convert force to hydraulic pressure comprising absolute pressure sensors are less expensive than load cells comprising gauge pressure sensors because the micromachined sensors themselves are less expensive and because absolute pressure sensors simplify the design of load cell because it is not necessary to provide a duct from the pressure sensor to the outside atmosphere. The output of a force sensor comprising an absolute pressure sensor responds to changes in atmospheric pressure. Going from sea level to an altitude of 5,300 feet at Denver, Colo. with the same occupant weight can cause indication of three to ten pounds decrease in the force sensed by each load cell.
It is well known to connect a sensor using only two electrical conductors. In typical designs the sensor simultaneously draws power needed to operate and also draws constant or pulsed current over and above the current it requires to operate. The additional current indicates the physical measurement.
Child seats are made in several types. Infant seats are mounted in a rear facing orientation and are typically intended for infants weighing Less Than 18 pounds. Child seats are mounted in a forward facing direction. They are usually intended for children weighing 18 and 40 pounds but some designs may be mounted in a rear facing orientation for use as an infant seat. The aforementioned two types provide their own seat belts and are anchored by the vehicle seat belts which are kept away from the child. The vehicle seat belts may be under substantial tension. The third type of child seat uses the vehicle seat belts to restrain the child and some are designed for children weighing as much as 60 pounds. Operation of the third type with a large belt tension is unlikely because of the discomfort it would cause.
U.S. Pat. No. 6,259,167 issued to the present inventor describes a seat occupant weight sensing system based on torque sensed at the cushion of a seat and two seat occupant weight sensing systems based on torque sensed at the frame of the seat.
U.S. Pat. No. 6,259,167 also discloses a force sensor comprising a liquid filled injection stretch blow molded bottle having bellows shaped sides and a pressure sensor thereby being a force sensor responsive to axial force. The force sensor operates by converting axial force to pressure in the liquid for sensing by the pressure sensor.
U.S. Pat. No. 6,224,094 issued to the present inventor describes a load cell for generating an electric signal indicating applied force. The load cell has a pressure sensor and a means for converting applied force to pressure whereby its output becomes a force signal. The load cell is preloaded by a constant force spring whereby relative thermal expansion between the liquid and the structural parts of the load cell does not cause the pressure in the liquid to vary. The spring also provides a low friction bearing in the axial direction and resists radial movement between two parts of the load cell.
Weight sensing systems comprising a platform and four load cells at the corners of the platform are well known. For example, U.S. Pat. No. 4,056,156 issued to Arnold J. Dayton on Nov. 1, 1977 teaches a bathroom scale having four load cells each having a resilient metal bellows for pressurizing liquid and connected to a common plenum connected to a pressure sensor. These weight sensing systems can be quite sensitive to temperature unless means are provided to accommodate the change of volume of the liquid with temperature. One exception is if the fluid is water at room temperature because water has a very low thermal expansion coefficient between 5xc2x0 C. and 25xc2x0 C. For vehicle occupant weight sensing, accurate response is required between xe2x88x9240xc2x0 C. and +100xc2x0 C. and for this operating temperature range all known fluids exhibit large thermal expansions relative to metals and most plastics.
Copending application Ser. No. 09/289,048 discloses a force sensor comprising a liquid filled injection stretch blow molded bottle having bellows shaped sides and a pressure sensor in a load cell having a disk spring whereby the load cell is preloaded by the force of the disk spring and the disk spring also operates as a bearing that allows axial movement but resists radial movement.
A general object of this invention is to provide a seat occupant weight sensing system offering low cost and superior performance and also to provide a load cell that is particularly adapted for sensing force derived from the weight of a seat occupant which also overcomes certain disadvantages of the prior art.
In accordance with the invention, a seat occupant weight sensing system comprises four load cells located in the force path between the seat occupant and the vehicle structure. Each load cell comprises two input members and a pair of springs that operate in concert to isolate force applied in the direction of an axis of the load cell from other forces. This enables a force sensor to respond to the isolated axial force and not respond to the other forces.
Further, in accordance with the invention, the two springs are conical springs under a stress that makes them flat. Cylindrical flanges are provided at the inside diameters and the outside diameters of the springs. The conical springs are affixed to the input members only at the ends of the flanges which substantially eliminates friction when the input members move relative to each other. The conical springs also prevent radial movement between the input members of the load cell.
Further, in accordance with the invention, the two springs resist axial misalignment between the input members. This causes the load cell to compel the normal parallel alignment between the part of the vehicle floor to which the seat attaches and the part of the seat that is attached to the vehicle or between other elements joined by the load cells.
Further, in accordance with the invention, the mounting between the load cell and an element to which the load cell is mounted is adapted to yield in a sideways direction when subjected to a modest side force such as 50 pounds. The yielding limits the side force applied to the load cell to approximately fifty pounds. Limiting the side forces to which the load cell is exposed eases the requirements for insensitivity to side forces relative to a load cell that might have to withstand the substantial side forces which might result from such as relative thermal expansion between two vehicle components between which a load cell is attached.
Further, in accordance with the invention, the load cell comprises means for converting force applied to the input members to pressure in a fluid and a pressure sensor providing an electric signal indicating the hydraulic pressure.
Further, in accordance with the invention, the fluid has a much larger thermal expansion coefficient than the materials of which the other parts of the load cell are made, and the difference in thermal expansion coefficients between the fluid and the other parts is compensated by using designs and materials that provide partial or complete compensation that reduces or eliminates the variation of the output of the load cell with temperature. It advantageously happens that the materials that compensate for differential thermal expansion are also particularly desirable materials for the functions they perform in the load cell.
Further, in accordance with the invention, an input member is attached to a vehicle seat or the vehicle structure by a threaded fastener and isolation means are provided to prevent distortion of the input member by stresses resulting from tightening the threaded fastener from affecting the output of the force sensor.
Further, in accordance with a first embodiment of the invention, a multiplicity of load cells each includes a pressure sensor and each receives some of the weight to be measured. The load cells are connected with a common circuit which adds the outputs of the load cells to calculate the total seat occupant weight.
Further, in accordance with a second embodiment of the invention, a multiplicity of load cells are provided, each having a chamber containing pressurized fluid. The chambers are in fluid communication with a common plenum whereby all the fluid chambers are at the same pressure. A single pressure sensor measures the plenum pressure which indicates the sum of the axial forces applied to the load cells.
Further, in accordance with the aforementioned second embodiment of the invention, inherent temperature compensation of each load cell causes the output of the single pressure sensor to accurately indicate the sum of the axial forces even when different load cells are at different temperatures.
Further, in accordance with the invention, an atmospheric pressure sensor informs the microprocessor of the atmospheric pressure which enables the pressure sensors of the load cells of the invention to be absolute pressure sensors and enables the microprocessor to correct for variations in the outputs of the load cells caused by variations in atmospheric pressure.
Further, in accordance with the invention, a switch responsive to seat belt tension is provided that closes at a predetermined tension that would be uncomfortable to a human and therefore indicates that the seat is being occupied by a tightly belted child seat.
Further, in accordance with the invention, a seat belt tension sensor comprises two bowed bands adapted to straighten when stressed by seat belt tension. A sensor responds to the straightening of the two bands. An extension from the seat belt applies tension to the bands and protects against torques that might tend to unevenly stress the bowed steel bands. A microprocessor uses the tension to calculate the weight of the seat occupant in the presence or absence of substantial seat belt tension.
A complete understanding of this invention may be obtained from the description that follows taken with the accompanying drawings.